The present invention relates to an improvement of an air-fuel ratio control circuit for an internal combustion engine, and more particularly to measures against noises in an internal combustion engine which circuit is adapted for driving an air-fuel ratio control pulse motor under control in response to the output signal from an exhaust gas sensor, such as an O.sub.2 sensor.
As the output signals from an O.sub.2 sensor contain high frequency noises caused by the ignition and others, it has previously proposed to transmit the output through a low-pass filter LPF1 composed of a resistor R1 and a capacitor C1 to a comparator CM1 composed of resistors R2 and R3 and an operational amplifier OPA1, as is shown in FIG. 1. The waveform shown in FIG. 2A which contains a noise NA is transmitted from the sensor to the filter LPF1 from which the waveform shown in FIG. 2B is outputted, containing the noise waveform NB. When the reference voltage Vr of the comparator CM1 is set above the voltage of the noise waveform NB, for example at Vr1 indicated by the chain line in FIG. 2B, the original output waveforms PA2 and PA4, for example, in FIG. 2A are cut off together with the noise waveform NA, and the pulse duration of the output from the comparator CM1 becomes shorter than that from the sensor, and consequently the output from the comparator CM1 cannot correspond to that from the sensor. When the reference voltage Vr of the comparator CM1 is, however, set to Vr2 indicated by the solid line in FIG. 2B, almost no waveform of the output from the O.sub.2 sensor is cut off, and disadvantageously the noise waveform NA in FIG. 2A will be also transmitted in the form of NC in FIG. 2C from the comparator CM1. The pulse durations of the output pulses PC1 to PC4 from the comparator are shorter than those of the output pulses
to PA4 in FIG. 2A from the sensor, and consequently more or less the output from the comparator CM1 cannot precisely correspond to that from the sensor.